The present invention relates to power supply circuits, and more particularly, to power supply circuits for supplying power to alternating-current (AC) loads, for example lighting dimmer circuits, and wherein the circuit employs a power supply for providing power to a control circuit controlling a switching circuit supplying power to the load and which protects the load in the event of switch failure.
Circuits for providing variable power to AC loads are known, for example, lighting dimmers. Some lighting loads are low-voltage lighting loads, which are supplied with AC power via a step-down transformer, typically an isolation transformer. These step-down transformers step the voltage down to the low-voltage level, for example 12 to 24 volts, necessary to power the lamp or lamps. A problem with low-voltage lighting loads employing a transformer, such as magnetic low-voltage (MLV) lighting, is that the transformers are susceptible to any direct-current (DC) components of the voltage across the transformer. A DC component in the voltage across the transformer can cause the transformer to generate acoustic noise and to saturate, increasing the temperature of the transformer and possibly creating a fire hazard.
In many countries, there are requirements that such magnetic low-voltage lighting loads incorporate thermal protection to protect against overheating. For example, some magnetic low-voltage lighting loads employ thermal sensors or fuses that trip in the event of an over-current condition to prevent overheating and fire hazards. However, this is not a universal requirement for magnetic low-voltage lighting loads, and accordingly, it is important to ensure that such magnetic low-voltage lighting loads, particularly where the loads are not thermally protected, are prevented from overheating.
Dimmer circuits utilize semiconductor switches, such as triac and field effect transistors (FETs), to control the power supplied to the lighting load. Since a triac is bidirectional device, if the triac fails shorted, current will flow in both half-cycles and no substantial DC component will be supplied to the load. Thus, the problem of overheating an MLV transformer due to a DC component of the voltage is not created. The end user of the dimmer will know that there is a problem with the dimmer because the connected lighting load will be on at full brightness and the user will not be able to dim the light. Of course, if the dimmer switch fails open there is no overheating problem as the load will not be supplied with power.
A problem does arise, however, with dimmers that employ FETs as the controlled switching devices. Individual FETs are not bidirectional switches, so generally two FETs are employed in an anti-serial connection, i.e., they are connected in series such that the sources of the two transistors are connected together such that they function as a bidirectional switch. FETs are often used in dimmers because they provide better EMI (electromagnetic interference) performance and more flexible control of the current through the load. In dimmer circuits employing FETs, power will flow through both transistors to the lamp load. In particular, in one half-cycle of the AC source current, the power will flow through the drain-source path of a first transistor (with the gate being appropriately controlled to provide the desired dimming level) and through the body diode and/or anti-parallel connected external diode connected across the second transistor. In the other half-cycle, the current will flow from the second transistor's source to the drain (with the gate controlled to provide the desired dimming) and through the body diode of the first transistor and/or the anti-parallel connected external diode.
If both of the series-connected FETs fail shorted, the situation is the same as when the triac fails shorted. The lamp load will be at full brightness and there will be no dimming and because both half-cycles are passed substantially completely, the lamp load will be at substantially full brightness. In two-wire dimmers (i.e., dimmers without a neutral connection), typically a small portion of the AC power is removed from the AC line source to power the dimmer control circuitry by obtaining power across the dimmer when the switches are off, i.e., during the phase cut portion of the power provided to the lamp load or before the switches conduct. This is because there is no neutral connection to the dimmer. With both the switches shorted, the control circuitry for the switches will not be provided with power. However, there is no overheating hazard because there is no DC component provided to the magnetic low-voltage lamp load, since both half-cycles are passed substantially equally.
If both the FETs fail open, no power is provided to the load and there is no overheating hazard.
The problem of overheating an MLV transformer occurs when only one of the FETs fails. In such case, should one FET fail shorted, the shorted FET will provide power to the load during the complete half-cycle. In absence of the failure, the FET normally would be able to control the power delivered to the load during the half-cycle. When one FET is shorted, the diode of the other FET will conduct since it is forward biased. The other FET will be controlled by the dimmer control circuit such that it provides a phase cut dimming signal during its half-cycle of conduction; and of course, the shorted switch will also conduct. Due to the asymmetry between the two half-cycles, a DC component will be provided to the load, thus creating an overheating hazard. If no thermal protection is provided, a potential fire hazard exists.
Should one of the switches fail open, there may be an asymmetry if the open failure leaves the body diode (or external diode) intact because in that case one half-cycle will be absent whereas the other half-cycle will be present, also causing an overheating hazard. If the open failure results in both the drain-source path and the body diode (or external diode) of one switch being open, no power can be provided to the load and there is no overheating hazard.
There is a need for a protection circuit in such dimmers to prevent the described overheating hazard due to the DC component in the case of switch failure, particularly in the case of magnetic low-voltage lamp loads, which at the same time ensures that even in the event of such a failure, power is provided to the power supply control circuit for the dimmer to enable the control circuitry of the dimmer to continue to operate in a manner so as to reduce or eliminate the DC component.